1. Field of the Inveniion
The present invention relates to a method of producing a ferroelectric thin film by chemical vapor deposition.
2. Description of the Prior Art
A ferroelectric substance has been practically used in many fields in view of the pyroelectric, piezoelectric and ferroelectric properties thereof, or the electro-optic effect thereof.
In many cases, single crystalline substances, such as glycine sulfate (TGS), LiTaO.sub.3 and LiNbO.sub.3, and sintered ceramics, such as BaTiO.sub.3, PbTiO.sub.3, PbZr.sub.x Ti.sub.1-x O.sub.3 where x is the number of zirconium atoms, and PbLa.sub.x Zr.sub.y Ti.sub.1-(x+y) O.sub.3 (PLZT) where x is the number of lanthanum atoms and y is the number of zirconium atoms, have been used as materials of a ferroelectric substance. In many cases ot producing a device from these materials, the materials are cut and polished into a thick film of 50 to 500 .mu.m.
However, single crystalline substances are generally difficult to produce, expensive and difficult to convert to a film due to the cleavage thereof.
On the other hand, ceramic porcelains have problems in that since they are generally fragile, many cracks are produced in the working process; it is difficult to obtain a thin film having a thickness of 50 .mu.m or less, requiring a high level of working technique and much time; and they have high production cost.
In view of the above, it has been strongly desired to establish a method of producing a thin film of 1 to 30 .mu.m thickness, with the objects of improvement in performance, miniaturization, multiple functionalization, provision of high performance, reduction of cost, and the like, for a device using a ferroelectric substance.
For example, the high-frequency spattering method or the magnetron spattering method with sintered bodies such as PbTiO.sub.3, PZT and PLZT as a target has been known as a method of producing a ferroelectric thin film made of PbTiO.sub.3, PZT, PLZT and the like.
The chemical vapor deposition method has been known as another method of producing the above described ferroelectric thin films. This method has developed as a method of forming a surface-protecting film, such as SiO.sub.2, Si.sub.3 N.sub.4 and Al.sub.2 O.sub.3, and a gate-insulating film of MOSFET in the semiconductor and electronics industries, and at present it is an important process of producing a semiconductor device. Also, it has been used as a method of forming a surface-coating film on heat resistant, corrosion resistant and abrasion resistant parts.
As a method of producing a ferroelectric thin film by the chemical vapor deposition method, there are known a method of producing a thin film of PbTiO.sub.3, in which water vapor and oxygen act upon lead chloride and titanium tetrachloride in the gaseous phase, and a method of producing a thin film of PbTiO.sub.3 by the reaction of titanium tetrachloride, lead thenoyltrifluoroacetone or acetylacetonate and water vapor among themselves in the gaseous phase.
Of the above described conventional methods of producing a ferroelectric thin film the high-frequency spattering method and the magnetron sputtering method have the problem that the deposition rate of a ferroelectric substance is small, i.e. to an extent of 10 to 50 .ANG./min, and it takes a long time to obtain a film having the desired thickness. In addition, since constituent atoms of sintered bodies used as a target are different in spattering coefficient, the ferroelectric thin film which is formed has a different composition than the target, and it is difficult to control the composition of the thin film to the stoichiometric value. Many other problems also occur in that, for example, damage is caused and defects are produced in the substrate surface and the resulting film owing to high-energy atoms and ions during the spattering.
As to the case where the chemical gas phase growth method is used, in a method in which lead chloride and titanium tetrachloride are used, since lead chloride is solid at normal temperature and has a small vppor pressure, a high temperature of 700.degree. C. is required for the vaporization thereof, and since it is difficult to control the vapor pressure of lead chloride, the resulting PbTiO.sub.3 thin films show different compositions. Also, problems occur in that, for example, since the resulting thin film contains lead chloride, it is difficult to produce a pure PbTiO.sub.3 thin film whereby a PbTiO.sub.3 thin film have a superior performance as a ferroelectric thin film can not be obtained.
Furthermore, the chemical vapor deposition method, in which titanium tetrachloride and lead thenoyltrifluoroacetone or lead acetylacetonate are used, has a problem in that the vaporization of organo-lead compounds requires 180.degree. to 220.degree. C., and the organo-lead compounds are gradually decomposed at this temperature. Accordingly, the composition of the gaseous mixture is changed with the lapse of time, whereby it is difficult to control the chemical composition of the objective PbTiO.sub.3 to a constant value.
Also, in the above described method using the chemical vapor deposition method, since titanium tetrachloride is used as a raw material for titanium oxide in both examples, the apparatus and the object substrate are corroded owing to the corrosive property of the resulting HCl gas, so that a problem also occurs in that the material of the substrate is limited to a material which is resistant to high-temperature, HCl-containing water.